I have written a very simple app that populates an NSBrowser bound to
an NSTreeController with a tree of NSTreeNode objects that is 2 levels
deep. The app can be made to crash (at least on my system) by
grabbing the bottom right corner of the window and pulling left to
narrow the window as far as possible (you may need to expand and
contract a few times to trigger the crash).

Given the linked code (also summarized below) what could be causing
this crash? If anyone has time to perhaps build this project and have
a look, I would be very grateful.

Here are the key lines of code, without which neither the data will
show up in the NSBrowser, nor will it crash: